Known in the art is a continuous-casting process for producing high-strength magnesium cast-iron castings, effected by feeding a magnesium cast iron batch-wise into a metal receptacle of a continuous-casting plant under a layer of protecting slag, provided on the surface of a molten cast-iron batch, containing 20-30% of magnesium chloride (SU, A, 944761).
Said process fails to ensure highly stable uniformity of the physico-mechanical properties of the casting in the course of continuous casting because of burning losses of magnesium during the melt holding.
Furthermore, realization of said prior-art process brings about contamination of the shop atmosphere with harmfull substances because of evolution of chlorine from magnesium chloride at high temperatures.
In addition, the above process cannot be automated, since it involves the operations of melting protecting slag, crushing, storing, and metering magnesium additions and slag, which do not lend themselves to automation.
A continuous-casting process for producing high-strength magnesium cast-iron castings is known (X Vsesoyuznaya konferentsiya po vysokoprochnomu chugunu, tezisy dokladov, Akademiya nauk Ukr.SSR, Kiev, lvov, 1977, pp. 110-111), residing in that, with a view to obtaining globular graphite in the structure of cast-iron castings, molten cast iron containing magnesium is poured into the metal receptacle.
During the holding of the magnesium cast iron in the metal receptacle contacting of said cast iron with the atmosphere causes burning losses of magnesium, which proceed in accordance with the reaction: EQU Mg.sub.vapour +1/2 O=MgO.
As a result, the quantity of magnesium in the melt diminishes continually, this leading to deterioration of the strength characteristics of the metal and to their appreciable non-uniformity along the length of the casting in the course of continuous casting.
Besides, said process leaves out of account changes in the content of magnesium in cast iron (burning losses of magnesium) upon holding cast iron at a high temperature.
Burning losses of magnesium bring down its content and upon attaining a certain minimum content permissible for given conditions (less than 0.03%) graphite crystallizes in a lamellar rather than globular form, that is, the structure of grey iron is obtained.
The strength characteristics of such a cast iron change considerably along the length of the casting (from the properties of high-strength cast iron to those of grey iron).
In addition, this process requires the operations of crushing, storing, and proportioning of magnesium additions into molten cast iron, which cannot be carried out in an automatic mode.
Widely known are methods of desulphurizing cast iron by injecting powdered magnesium into it in a stream of gases (N. A. Voronova. "Desul'furatsiya chuguna magniem", 1980, "Metallurgiya", Moscow, p. 102).
These methods enable the introduction of powdered magnesium into molten cast iron for the formation of globular graphite in the structure of the latter.
These methods, however, cannot be applied in a continuous-casting process for producing high-strength magnesium cast-iron castings, since they require considerable masses of treated cast iron.
To make the continuous-casting process trouble-free, the height of metal in the metal receptacle above the mould is maintained at 300 to 500 mm.
When immersing a tuyere into the melt to such a depth for injecting magnesium into the melt, the coefficient of magnesium utilization by the cast iron melt is very low (less than 15%).
The efficiency of the continuous-casting process of more than 9.5 kg/s requires a high consumption of magnesium and gas, this bringing about splashes of metal from the receptacle and deterioration of the working conditions of the service personnel.
Furthermore, injection of magnesium into molten cast iron involves complicated technological operations.
Also known is a continuous-casting process for producing high-strength magnesium cast-iron castings (SU, A, 544063), residing in a periodic feeding of magnesium cast iron into a metal receptacle, shaping a casting in a mould, and drawing the casting from the mould.
For ensuring stable quality of the material of the casting, the content of magnesium in the cast iron being replenished is increased by 0.01-0.1 mass % compared with the content of magnesium in the cast iron left in the metal receptacle by the moment of replenishing. Magnesium content in the cast iron being replenished is found from the relation: ##EQU1## where Mg.sub.2 is the content of magnesium in the cast iron being replenished, mass %;
.DELTA.Mg are burning losses of magnesium in the metal receptacle per unit of time, %; PA1 t is the time interval between two successive replenishings; PA1 p.sub.1 is the mass of cast iron left in the metal receptacle at the time of replenishing; PA1 p.sub.2 is the mass of cast iron being replenished; PA1 Mg.sub.1 is the content of magnesium in the cast iron left in the metal receptacle by the moment of replenishing, mass %.
But the use of said method in continuous production of castings from high-strength cast iron leads to weakening of the globularizing treatment effect upon overholding of cast iron in the metal receptacle because of burning losses of magnesium. Ultimately, the strength characteristics of the castings become impaired. Depending on the profile of the casting being drawn and on the thickness of its walls, the efficiency of the process varies within a wide range, the time of adding subsequent portions of magnesium cast iron (replenishing time) varies within a wide range accordingly, and may reach 0.5 h and more. During this period the burning losses of magnesium become considerable. Therefore, the properties of cast iron in the castings obtained before and immediately after the replenishing prove to be substantially non-uniform and display appreciable differences.
As a result, it is not feasible to produce castings with a high degree of uniformity of their properties over the length of the products being drawn in the process of continuous casting.
Moreover, treatment of the replenished portions of cast iron with magnesium in the ladle brings about a pyroeffect which leads to pollution of the shop atmosphere with noxious gases and to deterioration of the working conditions of the service personnel.
Magnesium content in the material of the casting, alongside of other parameters, determines the strength properties of the metal. A diminution of the magnesium content in the material of the casting to less than 0.03 mass % leads to a sharp decline in the strength characteristics.
Furthermore, the application of said method requires an operative control over the mass of magnesium cast iron left in the metal receptacle, over the quantity of magnesium in it, over the mass of the cast iron portion to be replenished, as well as over the quantity of magnesium in it, over the crushing and proportioning of magnesium additions. These operations interfere with the automation of the process.